Silica-shelled single quantum dot micelles

Quantum dots might become
more widely used in biological applications if better methods for making them water-soluble
were available. Chemically coating quantum dots to improve their water solubility
often makes them so large that they interfere with Förster resonance energy
transfer or normal cellular processes.

Silica-coated quantum dots are small and have
a high surface area and a large pore size, the latter of which may be useful for
biosensing and photodynamic therapy (PDT). However, silica coating often yields
quantum dots that quench fluorescence because they aggregate, thereby reducing the
quantum yield.

Researchers at the National Institute
for Advanced Industrial Science and Technology in Tosu, Japan, have modified the
silica-coating method to enable the encapsulation of one quantum dot in a silica
sphere by consecutively using three different silica precursors without exchanging
hydrophobic coordinating ligands. The product can be conjugated with biological
molecules.

In the Aug. 15 issue of Analytical Chemistry, they said that their silica-coating method yielded
quantum dots of uniform size — 17.42 ±2.12 nm in diameter with about
a 12 percent size distribution. The quantum dots had a high quantum yield and sharp
photoluminescence spectra with a full width half maximum of about 30 nm. Ninety-two
percent of the silica shells contained a single quantum dot, as the researchers
desired.

Before silica-shelling, they coated the quantum
dots with detergent micelles, which enable the incorporation of other hydrophobic
molecules, such as lipid-soluble photosensitizers, and hydrophobic oxidizing and
contrast agents. As a result, this technique can be used to fabricate biological
probes that have multiple functions. The researchers demonstrated that ability by
creating a probe with paramagnetic properties that enable its operation in magnetic
resonance and fluorescence imaging.

Experiments in which the quantum dots
were incubated with cell lines for 48 hours showed that they were potentially nontoxic.
Their small size and hydrophilicity could enable them to easily travel through the
urinary system, which the investigators are currently verifying. In addition, initial
studies showed that PDT performed with silica micelles containing quantum dots and
phthalocyanine could kill cancer cells.